Apache Tomcat Development

Class Loader HOW-TO

Table of Contents

Overview

Like many server applications, Tomcat installs a variety of class loaders
(that is, classes that implement java.lang.ClassLoader) to allow
different portions of the container, and the web applications running on the
container, to have access to different repositories of available classes and
resources. This mechanism is used to provide the functionality defined in the
Servlet Specification, version 2.4 — in particular, Sections 9.4
and 9.6.

In a Java environment, class loaders are
arranged in a parent-child tree. Normally, when a class loader is asked to
load a particular class or resource, it delegates the request to a parent
class loader first, and then looks in its own repositories only if the parent
class loader(s) cannot find the requested class or resource. Note, that the
model for web application class loaders differs slightly from this,
as discussed below, but the main principles are the same.

When Tomcat is started, it creates a set of class loaders that are
organized into the following parent-child relationships, where the parent
class loader is above the child class loader:

Bootstrap
|
System
|
Common
/ \
Webapp1 Webapp2 ...

The characteristics of each of these class loaders, including the source
of classes and resources that they make visible, are discussed in detail in
the following section.

Class Loader Definitions

As indicated in the diagram above, Tomcat creates the following class
loaders as it is initialized:

Bootstrap — This class loader contains the basic
runtime classes provided by the Java Virtual Machine, plus any classes from
JAR files present in the System Extensions directory
($JAVA_HOME/jre/lib/ext). Note: some JVMs may
implement this as more than one class loader, or it may not be visible
(as a class loader) at all.

System — This class loader is normally initialized
from the contents of the CLASSPATH environment variable. All
such classes are visible to both Tomcat internal classes, and to web
applications. However, the standard Tomcat startup scripts
($CATALINA_HOME/bin/catalina.sh or
%CATALINA_HOME%\bin\catalina.bat) totally ignore the contents
of the CLASSPATH environment variable itself, and instead
build the System class loader from the following repositories:

$CATALINA_HOME/bin/bootstrap.jar — Contains the
main() method that is used to initialize the Tomcat server, and the
class loader implementation classes it depends on.

$CATALINA_BASE/bin/tomcat-juli.jar or
$CATALINA_HOME/bin/tomcat-juli.jar — Logging
implementation classes. These include enhancement classes to
java.util.logging API, known as Tomcat JULI,
and a package-renamed copy of Apache Commons Logging library
used internally by Tomcat.
See logging documentation for more
details.

If tomcat-juli.jar is present in
$CATALINA_BASE/bin, it is used instead of the one in
$CATALINA_HOME/bin. It is useful in certain logging
configurations

$CATALINA_HOME/bin/commons-daemon.jar — The classes
from Apache Commons
Daemon project.
This JAR file is not present in the CLASSPATH built by
catalina.bat|.sh scripts, but is referenced
from the manifest file of bootstrap.jar.

Common — This class loader contains additional
classes that are made visible to both Tomcat internal classes and to all
web applications.

Normally, application classes should NOT
be placed here. The locations searched by this class loader are defined by
the common.loader property in
$CATALINA_BASE/conf/catalina.properties. The default setting will search the
following locations in the order they are listed:

tomcat-i18n-**.jar — Optional JARs containing resource bundles
for other languages. As default bundles are also included in each
individual JAR, they can be safely removed if no internationalization
of messages is needed.

tomcat-jdbc.jar — An alternative database connection pool
implementation, known as Tomcat JDBC pool. See
documentation for more details.

tomcat-util.jar — Common classes used by various components of
Apache Tomcat.

tomcat-websocket.jar — WebSocket 1.1 implementation

websocket-api.jar — WebSocket 1.1 API

WebappX — A class loader is created for each web
application that is deployed in a single Tomcat instance. All unpacked
classes and resources in the /WEB-INF/classes directory of
your web application, plus classes and resources in JAR files
under the /WEB-INF/lib directory of your web application,
are made visible to this web application, but not to other ones.

As mentioned above, the web application class loader diverges from the
default Java delegation model (in accordance with the recommendations in the
Servlet Specification, version 2.4, section 9.7.2 Web Application Classloader).
When a request to load a
class from the web application's WebappX class loader is processed,
this class loader will look in the local repositories first,
instead of delegating before looking. There are exceptions. Classes which are
part of the JRE base classes cannot be overridden. There are some exceptions
such as the XML parser components which can be overridden using the appropriate
JVM feature which is the endorsed standards override feature for Java <= 8
and the upgradeable modules feature for Java 9+.
Lastly, the web application class loader will always delegate first for JavaEE
API classes for the specifications implemented by Tomcat
(Servlet, JSP, EL, WebSocket). All other class loaders in Tomcat follow the
usual delegation pattern.

Therefore, from the perspective of a web application, class or resource
loading looks in the following repositories, in this order:

Bootstrap classes of your JVM

/WEB-INF/classes of your web application

/WEB-INF/lib/*.jar of your web application

System class loader classes (described above)

Common class loader classes (described above)

If the web application class loader is
configured with
<Loader delegate="true"/>
then the order becomes:

Bootstrap classes of your JVM

System class loader classes (described above)

Common class loader classes (described above)

/WEB-INF/classes of your web application

/WEB-INF/lib/*.jar of your web application

XML Parsers and Java

Starting with Java 1.4 a copy of JAXP APIs and an XML parser are packed
inside the JRE. This has impacts on applications that wish to use their own
XML parser.

In old versions of Tomcat, you could simply replace the XML parser
in the Tomcat libraries directory to change the parser
used by all web applications. However, this technique will not be effective
when you are running modern versions of Java, because the usual class loader
delegation process will always choose the implementation inside the JDK in
preference to this one.

Java <= 8 supports a mechanism called the "Endorsed Standards Override
Mechanism" to allow replacement of APIs created outside of the JCP
(i.e. DOM and SAX from W3C). It can also be used to update the XML parser
implementation. For more information, see:
http://docs.oracle.com/javase/1.5.0/docs/guide/standards/index.html. For
Java 9+, use the upgradeable modules feature.

Running under a security manager

When running under a security manager the locations from which classes
are permitted to be loaded will also depend on the contents of your policy
file. See Security Manager HOW-TO
for further information.